A magnetic coupler device is used as an isolator or the like for transmitting digital signals or analog signals. For example, the magnetic coupler device is applied to an interface that connects a computer to a peripheral device of the computer, an interface that connects circuits having different potentials to each other, an interface in a relay transfer device on a communication network or the like.
If a signal is to be transmitted between the circuits having different potentials, it is necessary to provide an interface for electrically isolating input and output sides from each other and, at the same time, for passing the input signal through an insulator by some means and supplying the input signal to the output side. Generally, a method of passing the input signal through the insulator is roughly classified into three types. Namely, the three types are methods using light, magnetic field and electric field, respectively. As an optically coupled isolator using light, there is known a photocoupler. As a magnetically coupled isolator using a magnetic field, there is known a GMR isolator that employs a pulse transformer or a giant magneto-resistance (GMR) device. As an electric field coupled isolator using an electric field, there is known a capacitively coupled isolator that employs a very small capacitance of an insulator between input and output sides.
The isolators of these types are all insulating interfaces including both an electrically insulating function and a signal coupling function between the input and output sides. While coupling using the light is immune to the influence of the electric field or magnetic field from outside, coupling using the magnetic field or the electric field has a great improvement in a transmission rate from the coupling using the light.
The photocoupler is configured to mainly include a light emitting diode (LED) and a photodetector, and an input side and an output side of the photocoupler are electrically isolated from each other by resin. If current is applied to the LED, then the LED emits light and the light arrives at the photodetector via the resin. Frequency characteristics of the photocoupler are flat from DC up to a given frequency, and the transmission rate is decided depending on various characteristics of internal optical devices or the like and has its limits to several tens of Mbps in digital transmission.
The pulse transformer is to transmit a signal by electromagnetic induction between a primary coil and a secondary coil. The pulse transformer exhibits high transmission efficiency and can realize two-way communication. As for frequency characteristics, the pulse transformer cannot transmit DC for the following reasons. The secondary coil detects a magnetic field change generated by a current change of the primary coil as a current change. Accordingly, the pulse transformer cannot transmit a DC signal that is not accompanied by current change. It is considered that a limit to acceleration of the pulse transformer depends on a magnetic material of a core. Moreover, a transmission frequency band is defined to be up to 100 MHz in a currently popular gigabit LAN according to standards. Therefore, to realize one Gbps, four lines each at a transmission rate of 250 Mbps are employed and multi-leveling (to five levels) is carried out per line. Namely, if an operating rate of the pulse transformer used in the network is applied to the transmission frequency band of the gigabit LAN, the operating rate is about 125 Mbps for binary digital transmission.
The GMR isolator can be regarded as a device obtained by replacing the secondary coil of the pulse transformer by a magnetic field sensor that employs a GMR device. Since the GMR device detects a magnetic field intensity change generated by an input current change as a resistance value change, the GMR isolator can transmit a DC signal. Although it is basically difficult for a transmission rate of the GMR isolator to exceed that of the pulse transformer, the GMR isolator can realize the transmission rate of 100 Mbps in digital transmission. Because of flat frequency characteristics of the GMR isolator from DC up to a given frequency as compared with those of the pulse transformer, the GMR isolator is considered to be a high-rate isolation device that can replace the photocoupler and can be expressed as “magnetic coupler” in a narrow sense.
The capacitively coupled isolator is to transmit a signal through a small capacitance of the insulator between the input and output sides. Since the same path is shared between the signal and noise, it is necessary to set a frequency band of the signal higher than that of the noise. Namely, the small capacitance of the isolator makes it easy to pass the signal through the isolator and difficult to pass the noise through the isolator. Accordingly, frequency characteristics of the capacitively coupled isolator are limited to a high frequency band and the capacitively coupled isolator cannot transmit DC. The capacitively-coupled isolator realizes a transmission rate of 150 Mbps in digital transmission.
One of backgrounds of need to accelerate transmission rates of these insulating interfaces is as follows. Because high-rate microcomputers, DSP and FPGA emerge to follow development of semiconductor technology, accuracy for device control is increasingly improved and acceleration is increasingly improved. On the other hand, high-rate microcomputers become noise sources and have growing influence on peripherals (such as an analog circuit). Accordingly, it is desired to satisfy both acceleration of the interface device and enhancement of isolating properties of the interface device so as to improve the accuracy and to accelerate transmission rates of devices in the future.
To realize the acceleration in digital transmission, it is necessary to reduce an S/N ratio, carry out multi-leveling and widen the transmission frequency band. On the other hand, it is necessary to solve the problem that DC signals cannot be transmitted. There is no need to transmit a DC signal if the pulse transformer is employed in the network. However, for other usages of the pulse transformer, it is required to transmit digital signals as they are without encoding them if digital signals at the same level are continuously transmitted for certain time, or it is required to transmit an analog signal waveform as it is. To realize the requirements, it is necessary to transmit DC signals. If DC signals are to be transmitted using the capacitively coupled isolator, a transmission method by converting each DC signal into a pulse width modulated (PWM) signal may be used. However, in this case, it is necessary to separately construct a circuit system for conversion. In such usages, the GMR isolator is advantageous over the other types of isolators because DC signals can be transmitted and a frequency band of the GMR isolator is wide.
As a conventional GMR isolator, there is known a GMR isolator configured so that an electrostatic shield is provided between a spiral input coil and a magnetic field sensor that employs a GMR device and the electrostatic shield is grounded, thereby reducing floating capacitance between the input and output sides (see Patent Documents 1 and 2). This technique can suppress noises generated on output sides if voltage suddenly changes between the input and output sides. Namely, the conventional GMR isolator uses the fact that capability to eliminate common mode signals between the input and output sides can be improved.
The problem with the GMR isolator is that the transmission rate cannot be accelerated to be equal to or higher than 100 Mbps. If the transmission rate is accelerated, a high frequency signal carried across the input coil generates noise in an output-side magnetic field sensor due to induced voltage. The electrostatic shield between the input coil and the magnetic field sensor can suppress the noise due to the induced voltage. However, since the electrostatic shield is a conductor, a magnetic field from the input coil generates eddy current in a direction of canceling magnetic field change. As a result, the magnetic field disadvantageously attenuates. If the magnetic field attenuates, a quality of a signal waveform is degraded by a reduction in the S/N ratio following lowering of signal level, which possibly causes circuit malfunction.
Furthermore, the conventional GMR isolator has the following problems in arrangement, structure and the like of the input coil and the magnetic field sensor if designed for higher frequency. As shown in FIG. 20, the conventional GMR isolator includes a magnetic coupler device 103 configured to include an input coil 101 and a Wheatstone bridge that includes a GMR device serving as a detection circuit 102, and a differential receiver (differential amplifier) 104 calculating a difference between two outputs from the magnetic coupler device and amplifying the difference. The magnetic coupler device 103 can be regarded as a device having one input port and two output ports. The differential receiver 104 calculates the difference between two differential signals output from the two output ports and opposite in phase in the detection circuit 102, that is, the Wheatstone bridge, thereby making it possible to reduce common mode noise. However, the input port is connected to each of the output ports not only magnetically but also capacitively and inductively. Therefore, if frequency is higher, a difference of impedance between the input port and one of the two output ports and that between the input port and the other output port is greater and asymmetric normal mode noises that the differential receiver 104 cannot eliminate are output to the two output ports, respectively.
A transmission rate C (bit/sec.) in digital transmission is decided by a frequency bandwidth B and a signal-to-noise ratio (S/N ratio) according to Shannon's theorem expressed by the following Equation (1).
                    C        =                  B          ⁢                                          ⁢                                    log              2                        ⁡                          (                              1                +                                  S                  N                                            )                                                          (        1        )            
According to this Equation (1), if the S/N ratio is high, the transmission rate can be accelerated by multi-leveling transmission. Considering how much the frequency band should be secured for a device having a fixed transmission rate, if the transmission rate C is set constant, the frequency band B can be suppressed by multi-leveling as long as the S/N ratio is high. Generally, however, if a transmission rate of a device is to be accelerated, binary digital transmission is basically adopted. In addition, to realize highly reliable digital transmission, the device is often developed while setting a larger bandwidth for the transmission rate. For example, if the binary digital transmission is performed by a pulse waveform, high reliability can be ensured by securing a transmission frequency band about three times as large as the transmission rate. Namely, to realize the transmission rate of 100 Mbps, a frequency bandwidth from DC to about 300 MHz is set as a guide.
If attention is paid to transmission frequency characteristics of the device, as a frequency is higher than around several tens of MHz, symmetry as a transmission circuit seen from two output terminals is gradually destroyed and noises resulting from the induced voltages are greater. The S/N ratio is reduced, accordingly. Considering these problems, there is proposed a method of increasing the transmission rate by multi-leveling while restricting the frequency band and securing the S/N ratio. Nevertheless, because of need to realize the highly reliable digital transmission, there is a limit to acceleration by multi-leveling. It is, therefore, necessary to widen the frequency band so as to accelerate the transmission rate.
Namely, if the transmission rate is equal to or lower than 100 Mbps, the device can be designed easily while ignoring asymmetry as a transmission circuit without influence of the noises resulting from induced voltages by suppressing the transmission frequency band. However, if the transmission rate is equal to or higher than 100 Mbps, limits occur to multi-leveling while suppressing the transmission frequency and, therefore, there is no avoiding widening the frequency band to higher frequency side. If the frequency band is widened to the higher frequency side, the influence of the noises resulting from two different induced voltages due to the asymmetry as the transmission circuit is gradually greater, accordingly, which thus disturbs acceleration of the transmission rate. Moreover, the noises resulting from the induced voltages can be reduced by inserting the electrostatic shield into the conventional GMR isolator but the asymmetry as the transmission circuit remains unchanged even by insertion of the electrostatic shield. As a result, if the transmission frequency band is widened to the higher frequency side to accelerate the transmission rate, there eventually comes a limit to reducing the noises by inserting the electrostatic shield.
Causes for occurrence of the induced voltages at the output terminals can be roughly classified into two types, that is, capacitive coupling and mutual inductance-related coupling. The former coupling derives from a path of floating capacitance between the input and output sides whereas the latter coupling is electromagnetic inductive coupling between an input-side conductor and an output-side sensor conductor. In reality, the floating capacitance between the input and output sides is left even if the electrostatic shield is provided. Moreover, as for the mutual inductance-related coupling between the input and output sides, overcurrent is generated in the output-side sensor conductor by applying an alternating-current magnetic field to the input side. It is considered that the induced voltages occur to the output sides, depending on circuit arrangement on the output sides. The influence of these causes for occurrence becomes greater if the frequency is higher.
Patent Document 1: Patent Application Publication No. 2001-521160.
Patent Document 2: Patent Application Publication No. 2003-526083.
Patent Document 3: Patent Application Publication No. 2000-516714.
Patent Document 4: Patent Application Publication No. 2005-515667.